为vagrant windows box更正“config.ssh.shell”值？

Vagrantfile的内容：

Vagrant.configure(2) do |config|
  config.vm.box = "win7_base_test"
  config.vm.network "forwarded_port",guest: 88,host: 3088
  config.ssh.username = "vagrant"
  config.ssh.password = "vagrant"
  config.ssh.port = "3088"
  config.ssh.host = "127.0.0.1"
  config.ssh.shell = "cmd"
end

结果我得到以下列表,最后出现错误：

vagrant up

Bringing machine 'default' up with 'virtualbox' provider...
==> default: Clearing any previously set forwarded ports...
==> default: Clearing any previously set network interfaces...
==> default: Preparing network interfaces based on configuration...
    default: Adapter 1: nat
==> default: Forwarding ports...
    default: 88 => 3088 (adapter 1)
==> default: Booting VM...
==> default: Waiting for machine to boot. This may take a few minutes...
    default: SSH address: 127.0.0.1:3088
    default: SSH username: vagrant
    default: SSH auth method: password
    default: Warning: Connection timeout. Retrying...
The configured shell (config.ssh.shell) is invalid and unable
to properly execute commands. The most common cause for this is
using a shell that is unavailable on the system. Please verify
you're using the full path to the shell and that the shell is
executable by the SSH user.

注意：我可以通过在此期间执行“vagrant ssh”连接到vagrant box并在那里发出shell命令.

因此我有一个问题：我对“config.ssh.shell”参数使用正确的值(“cmd”)吗？那里有什么价值？

谢谢！